Vitamin D(s) (hereinafter, simply abbreviated as vitamin D) is known to rigidly bind to a binding protein (DBP: a vitamin D-binding protein, also referred to as a Gc globulin) in blood. Thus, in order to precisely measure an amount of vitamin D by an antigen-antibody method, it is necessary to dissociate vitamin D from DBP (pretreatment). As such a pretreatment, an organic solvent (e.g., ethanol, methanol, DMSO) is used in addition to a denaturating agent (e.g., an acid, a protein denaturating agent, a surfactant, a hydrolytic enzyme) (Patent Literatures 1 to 7).